1. Field of the Invention
The present invention relates generally to ink jet printing technology, and is particularly concerned with techniques for suppressing residual ink vibration after ink droplet ejection from the ink jet head.
2. Description of the Related Art
In general, an ink jet head comprises a pressure generating ink ejection chamber for applying pressure to ink to selectively eject it therefrom. One end of the pressure generating chamber is typically connected to an ink tank through an ink supply path, and the other end connects to a nozzle opening from which the ink drops can be ejected. Part of the pressure generating chamber is made to be easily deformed and functions as a diaphragm. This diaphragm is elastically displaced or deformed by an electromechanical converter such as a piezoelectric or electrostatic driver to selectively generate the pressure that ejects ink drops from the nozzle opening.
Recording apparatuses using this type of ink jet head offer outstanding operating characteristics, including low operating noise and low power consumption, and are widely used as hard copy output devices for a variety of information processing devices. As the performance and functionality of information processing devices has improved, demand has also risen for even higher quality and speed printing both text and graphics. This has made urgent the development of technologies enabling even finer ink drops to be ejected consistently at even higher frequencies or print speed.
Because of the structure of the ink jet head as described above, vibration remains in the ink inside the pressure generating chamber (also called the ink chamber because it is filled with ink; hereafter "ink chamber") after ink ejection, and this residual vibration can easily result in the formation of undesirable ejected ink droplets (also called "satellites"). To avoid this, the conventional approach has been to increase the flow resistance of the ink supply path connecting the ink chamber and ink tank to alternate the residual ink vibration. However, if the flow resistance of the ink supply path is high, the ink refill supply rate of ink to the ink chamber after ink ejecting is reduced, thereby lowering the maximum ink eject frequency, and ultimately the printing speed of the printing device.
Alternatively, as described in JP-A-S56-161172 (1981-161172), residual vibration can be canceled, and satellite emissions thereby prevented, by applying at an appropriate timing after the diaphragm drive signal a complementary signal canceling the residual vibration of the diaphragm. This resolves the problem described above, at least for non-varying droplet applications, and achieves a recording apparatus with a high output speed.
However, with the technology described in JP-A-S56-161172 (1981-161172), the diaphragm must be driven at an appropriate timing determined by the specific vibration period of the ink vibration system in order to cancel the residual vibration of the diaphragm. This is because residual diaphragm vibration may actually be promoted if the cancel signal timing is inappropriate. The technology described in JP-A-S56-161172 (1981-161172) therefore provides a variable resistor for adjusting the signal timing according to the specific vibration period of the ink vibration system. The problem here is that a sufficient vibration damping effect may not be achieved when any of the parameters determining the specific vibration period of the ink vibration system, e.g., the ink viscosity, change as a result of environmental changes, typical of which are ambient temperature fluctuations.
Also, expressing various density gradations by changing the size of the ink droplets formed on the recording medium is a preferred means of improving print quality. The size of the ink droplets output by any recording apparatus (printer) using an ink jet head is determined by various factors, one of which is the size (also called "ink ejection mass") of the ink drops ejected by the ink jet head.
A technology providing plural electrostrictive means of different sizes in the ink chamber, and separately controlling and driving these electrostrictive means to eject ink droplets of various sizes, is described in JP-A-S55-79171 (1980-79171). But, when the technological concept described in JP-A-S55-79171 (1980-79171) is applied, each of the plural, different size actuators used to deform the diaphragm must be independently driven, increasing the number of wires needed, and thus making it difficult to achieve a high nozzle density. The number of drivers also increases because of the need to separately drive each actuator, and this makes it difficult to reduce the device size.